As an infrared-emitting element used for a gas analysis sensor or the like, an infrared-emitting element for emitting infrared rays from a filament by energizing the filament to make it generate heat has conventionally been used.
Reference numeral 101 in FIG. 22 denotes a conventional gas analysis system using an infrared-emitting element 102 with a filament.
More specifically, in this gas analysis system, infrared rays 141 emitted by the filament of the infrared-emitting element 102 are intermittently shielded by a chopper 142, and become modulated infrared rays 144, which enter a gas 143 via a filter 146.
The modulated infrared rays 144 transmitting through the gas 143 are received by a light-receiving element 145.
This gas analysis system 101 calculates the concentration of the gas 143 from the ratio of the maximum value to minimum value of the light-receiving power level upon receiving the modulated infrared rays 144.
Accordingly, the gas analysis system 101 requires the modulated infrared rays 144, and must comprise the chopper 142 because the infrared-emitting element 102 can emit only constant infrared rays.
In recent years, it is required that small-sized, low-cost gas analysis systems.
Development of infrared-emitting elements capable of modulated emitting infrared rays without using any chopper 142 is required.
Many infrared-emitting elements using a ceramic bulk material as a heat-generating element, a silicon micromachining, and the like have been developed.
In an infrared-emitting element using a ceramic bulk material as a heat-generating element, however, infrared rays cannot be modulated at a high frequency because the thermal conductivity of a high-temperature portion is small, and the heat capacity of the bulk material is large.
For example, when this infrared-emitting element emits infrared rays at 48 Hz, the difference between the lowest and highest temperatures at the heat-generating portion is only 150.degree. C. and hence the difference between the minimum and maximum emission quantities of infrared rays is small.
In an infrared-emitting element using a silicon micromachining, as described in the following reference, boron is thermally diffused as a p.sup.+ -type impurity into a silicon bridge structure to selectively etch and remove a sacrificial n-type layer. The p.sup.+ -type layer of the silicon structure is formed into a bridge-building structure, and the bridge-building portion is made to generate heat, thereby emitting infrared rays (reference: Technical Digest of the 11th Sensor Symposium, 1992, pp. 169-172, Kimura et al.).
In the infrared-emitting element using the silicon micromachining, since the p.sup.+ -type layer is formed by thermally diffusing boron, the bridge portion of the bridge-building structure is too thick, resulting in poor thermal response characteristics for the driving power.
If the bridge portion can be made thin, the thermal response characteristics for the applied electric power can be improved drastically.
However, when the bridge portion obtained by thermally diffusing boron is decreased in thickness to about the reciprocal of the absorption coefficient at a necessary infrared wavelength, the emissivity of infrared rays rapidly decreases to weaken the infrared emission intensity. At present, only an infrared-emitting element using a silicon micromachining with a bridge portion having a thickness of about 5 .mu.m is realized.
In the conventional infrared-emitting element using the silicon micromachining obtained by thermally diffusing boron as a p.sup.+ -type impurity, the thermal response characteristics are poor because of the thick bridge portion. If infrared rays are emitted under constant-voltage driving, an excessively large current may flow to fuse the bridge portion because a long time is needed to increase the resistance value by temperature rise upon applying the voltage.
To avoid this situation, a protective circuit must be arranged as a driving circuit for the infrared-emitting element, or a constant-current driving method must be employed. This complicates the arrangement of the driving circuit.
The conventional infrared-emitting element using the silicon micromachining obtained by thermally diffused boron as a p.sup.+ -type impurity poses the above problems because the concentration of boron and the diffusion profile cannot be independently controlled with high precision in thermally diffusion method.
For an infrared-emitting element of this type, the concentration of boron and activation of boron to serve for an impurity layer are important factors to promote the emissivity of infrared rays.
However, thermal diffusion leads to a low concentration of boron and weak activation of boron to serve for an impurity layer, so the bridge portion cannot be made thin. Heretofore, even if a thin bridge portion can be formed, the emissivity of infrared rays decreases.
In thermal diffusion method, a thickness of the bridge portion is limited by a mechanical safety after etching.